This invention relates to methods of and apparatus for marine seismic exploration, and more particularly to arrays of sources and receivers which have vertical directivity.
In marine seismic exploration the seismic energy can be generated by a linear array of sources which are towed behind the boat. The towing cables provide a fixed spacing between the sources. Typically, each source may be an "air gun" which releases pulses of compressed air into the water. A typical air gun is shown in U.S. Pat. No. 3,506,085 to George B. Loper. Such guns emit seismic pulses having a characteristic frequency including the lowest frequency, the highest frequency and the predominant frequency of the seismic energy in each pulse.
The boat also tows a streamer of hydrophones which detect the seismic energy reflected from subsurface formations. Systems which have a long offset between the sources and the hydrophones are subject to high amplitude nearly horizontal traveling source-generated noise.
It can be shown that there is a critical distance from the source which is determined by the acoustic velocity in the water and the acoustic velocity in the bottom. Beyond the critical distance, energy is totally reflected and propagates horizontally. It is desirable to use sources and receivers which discriminate against such horizontal propagation.
In the prior art the array length and the spacing between the elements of the array have been determined by the predominant frequency in the seismic pulse. Such techniques provide directivity for energy at the predominant frequency but they do not give good directivity over the broad band of the seismic pulse. For example, in U.S. Pat. No. 3,479,638, the spacing between the sources is an integer of one-half the wavelength of the desired frequency. In U.S. Pat. No. 3,613,823, the length of the array is greater than one wavelength of the predominant frequency. Such approaches will not give directivity over the broad band of the seismic pulse.